The Benthic Foraminifera Assemblage And Its Bottom Water Environment Recorded Since The Middle Pleistocene In The Andaman Sea

The South Asian monsoon is an important part of the Asian monsoon system.The Northeast Indian Ocean plays an important role in global climate change and can serve as a perfect area for studying the evolution of the Indian summer monsoon.The Andaman Sea is one of the less researched marginal seas in the world.The characteristics of sediments and their sedimentary processes have far-reaching significance in reconstructing the paleoenvironment and paleoclimate of the region.The surface morphology,composition and morphological characteristics of the benthic foraminifera population are controlled to different degrees in the marine environment.The analysis of benthic foraminiferal combination features and their characteristics in sediment samples is of great value for the reconstruction of ancient marine environments.In this paper,the sedimentary core of the upper 22 m U1448 station of IODP 353 voyage was selected.The chronostratigraphic framework was preliminarily established using the volcanic ash layer and age control points of biological events.The identification and statistics of benthic foraminosa species in sediment samples were combined with foraminifera.Based on the ecological characteristics of the assemblage the underlying water environment since the Middle Pleistocene in the Andaman Sea was analyzed,and the relationship between the bottom water environment and the evolution of the monsoon was preliminarily explored.Through systematic palaeontological studies on benthic foraminifera in 100 sediment samples from station U1448 in the Andaman Sea,a total of 134 species of65 genera were identified,and 45 species were systematically analyzed for paleontological analysis,descriptions and illustrations were made.Based on the combination of benthic foraminifera,the content of Globocassidulina subglobosa,a species rich in dissolved oxygen,and the U+B content of the oxygen-deficient species,it was found that full range of foraminifera was observed in the late-ice period MIS1,MIS3,MIS5 and MIS7.The abundance of benthic foraminifera are relatively high,indicating high paleoproductivity.However,Globocassidulina subglobosa,a marker species rich in dissolved oxygen in these stages,is the species with the highest content in the whole population and is the absolute dominant species.The assemblage of U+B content characteristics shows that the bottom water has a high oxygen content during the interglacial period and does not show an oxygen-deficient environment with high productivity.Contrary to the ice age,the abundance of foraminifera and the abundance of benthic foraminifera were all lower,but the bottom water showed an oxygen-deficient environment with low oxygen content.During the interglacial period,the monsoon of the Andaman Sea increased,and a large number of freshwater rivers flowed in.The Salween and Irrawaddy rivers in the north mainly led to a large amount of terrestrial nutrient supply,and the surface water productivity increased,and the ice age reversed.The benthic foraminifera assemblage shows the lack of oxygen during oxygen enrichment during the interglacial glacial period of the bottom water and hypoxia during the glacial period.It is speculated that the bottom water environment may be affected by Undercurrent activity from other areas,or due to hydrate overflow during the glacial period,requiring more the verification of the indicators needs further study.